The biological importance of electron-transfer proteins cannot be overstated since life could not exist in the absence of an efficient means of transporting electrons. The objections of this research is to understand on a structural level the mechanisms and control of biological electron transfer in two specific protein systems: the cytochromes c2 which contain heme prosthetic groups and the high-potential iron-sulfur proteins (HiPIPs) which contain 4Fe-4S clusters. Both types of proteins are involved in electron transport in the purple photosynthetic bacteria and similar mechanisms are thought to modulate the redox properties and electron transfer rates of their respective prosthetic groups. The specific aim of this work is to use a combination of site- directed mutagenesis experiments, x-ray crystallography and kinetic measurements to test the current explanations for the modulation of the redox properties of the hemes and the 4Fe-4S clusters. Initially the high resolution structures of the cytochrome c2 from Rhodopseudomonas capsulata and the two very different HiPIPs isolated from Rhodospirillum tenue and Ectothiorhodospira halophila will be solved. Subsequently the structures of selected "mutants" with modified redox potentials and kinetic properties will be determined. These bacterial systems were chosen because they have been well studied using both biochemical and biophysical techniques and because the amount of protein needed for these studies is readily isolated from the organisms. Taken together these studies should provide a model for understanding on a quantitative and structural level the modulation of bacterial electron transfer proteins in general. In addition, what is learned from these bacterial systems will be directly applicable towards understanding the properties of electron-transfer proteins in higher organisms.